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The Dynamics of Adsorption on Clean and Adsorbate-Modified Transition Metal and Metal Oxide Surfaces

Description: Research directed toward understanding the dynamical factors governing the adsorption of molecules typically involved in heterogeneous catalytic processes has been continued at Harvard. Adsorption is the first step in any catalytic process, and predictions of rates of adsorption are fundamental to calculations of rates of catalytic reactions. Often activation of the bonds within the molecule proceed via a molecular precursor, and the rate of activation is determined by competitive bond activation and desorption of this precursor. Thus predictive capabilities for the adsorption probabilities of hydrocarbons become important for understanding the rates of simple reactions involving alkanes, such as reforming. In this work we have focused on the dynamics of molecular adsorption of low molecular weight alkanes on single crystal surfaces of platinum group metals in order to understand how different physical properties of the metals affect different trapping (adsorption) probabilities of the alkanes. The overall objective of these studies was to make a quantitative comparison of the molecular probabilities of C{sub 2}-C{sub 4} alkanes on different metals in order to assess the effects of the structures of the different alkanes and the intrinsic differences of the metals on the reactivity of the alkanes. This work built on previous studies with platinum and palladium single crystal surfaces for which we were able to apply molecular dynamic simulations to quantitatively predict trapping probabilities of low molecular weight alkanes for palladium directly from measured values of the trapping of ethane (only) on Pt(111). The trapping probabilities for the alkanes are higher on Pd(111) due to a larger dissipation of energy to lattice vibrations upon collision, suggesting a general scaling of trapping with the lattice force constant for different metal surfaces, provided the surface structure is not too different. In this grant period we have thus focused on the dynamics of molecular adsorption of ...
Date: September 13, 2007
Creator: Madix, Robert J.
Partner: UNT Libraries Government Documents Department

Near-Edge X-ray Absorption Fine Structure Spectroscopy of Diamondoid Thiol Monolayers on Gold

Description: Diamondoids, hydrocarbon molecules with cubic-diamond-cage structures, have unique properties with potential value for nanotechnology. The availability and ability to selectively functionalize this special class of nanodiamond materials opens new possibilities for surface-modification, for high-efficiency field emitters in molecular electronics, as seed crystals for diamond growth, or as robust mechanical coatings. The properties of self-assembled monolayers (SAMs) of diamondoids are thus of fundamental interest for a variety of emerging applications. This paper presents the effects of thiol substitution position and polymantane order on diamondoid SAMs on gold using near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and X-ray photoelectron spectroscopy (XPS). A framework to determine both molecular tilt and twist through NEXAFS is presented and reveals highly ordered diamondoid SAMs, with the molecular orientation controlled by the thiol location. C 1s and S 2p binding energies are lower in adamantane thiol than alkane thiols on gold by 0.67 {+-} 0.05 eV and 0.16 {+-} 0.04 eV respectively. These binding energies vary with diamondoid monolayer structure and thiol substitution position, consistent with different amounts of steric strain and electronic interaction with the substrate. This work demonstrates control over the assembly, in particular the orientational and electronic structure, providing a flexible design of surface properties with this exciting new class of diamond clusters.
Date: November 27, 2007
Creator: Willey, T M; Fabbri, J; Lee, J I; Schreiner, P; Fokin, A A; Tkachenko, B A et al.
Partner: UNT Libraries Government Documents Department

Separation of Olefin/Paraffin Mixtures with Carrier Facilitated Membrane Final Report

Description: This document describes the results of a DOE funded joint effort of Membrane Technology and Research Inc. (MTR), SRI International (SRI), and ABB Lummus (ABB) to develop facilitated transport membranes for olefin/paraffin separations. Currently, olefin/paraffin separation is done by distillation—an extremely energy-intensive process because of the low relative volatilities of olefins and paraffins. If facilitated transport membranes could be successfully commercialized, the potential energy savings achievable with this membrane technology are estimated to be 48 trillion Btu per year by the year 2020. We discovered in this work that silver salt-based facilitated transport membranes are not stable even in the presence of ideal olefin/paraffin mixtures. This decline in membrane performance appears to be caused by a previously unrecognized phenomenon that we have named olefin conditioning. As the name implies, this mechanism of performance degradation becomes operative once a membrane starts permeating olefins. This project is the first study to identify olefin conditioning as a significant factor impacting the performance of facilitated olefin transport membranes. To date, we have not identified an effective strategy to mitigate the impact of olefin conditioning. other than running at low temperatures or with low olefin feed pressures. In our opinion, this issue must be addressed before further development of facilitated olefin transport membranes can proceed. In addition to olefin conditioning, traditional carrier poisoning challenges must also be overcome. Light, hydrogen, hydrogen sulfide, and acetylene exposure adversely affect membrane performance through unwanted reaction with silver ions. Harsh poisoning tests with these species showed useful membrane lifetimes of only one week. These tests demonstrate a need to improve the stability of the olefin complexing agent to develop membranes with lifetimes satisfactory for commercial application. A successful effort to improve membrane coating solution stability resulted in the finding that membrane performance loss could be reversed for all poisoning ...
Date: March 12, 2007
Creator: Merkel, T.C.; Blanc, R.; Zeid, J.; Suwarlim, A.; Firat, B.; Wijmans, H. et al.
Partner: UNT Libraries Government Documents Department

Spectroscopic Characterization of Intermediates in the Iron Catalyzed Activation of Alkanes

Description: The present report begins with a brief survey of recent hypervalent iron chemistry and mentions two previously reported ferrate papers funded by the DOE/BES grant. The focus is then shifted to the seven publications acknowledging support of the grant that have not been reported since the last Progress Report, DOE/ER/14340-9, was prepared. These papers deal with: (a) the successful use of an ATR element in a stopped-flow infrared spectrometer, (b) the rationalization of a depolarization of a LiClO4 solution in polyethylene oxide high polymer, (c) an analysis of several coupled ultrasonic relaxations observed in solutions of pentoses undergoing isomerization, (d) the combination of ultrasonic absorption and Raman scattering measurements to elucidate zinc thiocyanate solutions in water, (e) the use of NMR to determine stability constants when LiClO4:12-crown-4 is dissolved in acetonitrile and in methanol, (f) the possible existence of triple ions in low permittivity solutions, and (g) the properties of a high surface area ceria aerogel. Collectively, these papers illustrate advantages of bringing several modern experimental techniques to bear on complex chemical systems.
Date: May 28, 2007
Creator: Eyring, Edward M.
Partner: UNT Libraries Government Documents Department

Supplemental Material: Enthalpy of Solvation Correlations for Gaseous Solutes Dissolved in Linear Alkanes (C5 thru C16) Based on the Abraham Model

Description: This document includes supplemental material to an article titled "Enthalpy of solvation correlations for gaseous solutes dissolved in linear alkanes (C5 thru C16) based on the Abraham model," published in QSAR & Combinatorial Science.
Date: June 2007
Creator: Mintz, Christina; Burton, Katherine; Acree, William E. (William Eugene) & Abraham, M. H. (Michael H.)
Partner: UNT College of Arts and Sciences